The present invention relates to a method for drying wood. More precisely, the inventive method relates to a method for drying wood comprising steps to efficiently eliminate or reduce, from the drying gases, terpenes and other volatile organic compounds, which are forced out of the wood during the drying process.
Wood raw material such as bio-fuel, chips or fibres are dried before being used for energy production or for manufacturing wood based boards, such as particle boards or oriented strand boards. During the drying process, terpenes and other volatile organic compounds are aforced out. Such organic compounds are often referred to as VOCxe2x80x94short for Volatile Organic Compoundsxe2x80x94and they will be thus denoted in the following. VOC evaporate along with moisture for instance, into the gases which during the drying process flow through the drying device and the wood material, and leave the drying device along with said moisture containing waste gases from the process. Normally the temperature of the waste or process gases vented during the drying process is between 80xc2x0 C.-150xc2x0 C., depending on the residual moisture contents of the wood material. Depending on the drying conditions, and to some extent on the composition of the wood raw material, essentially all VOC evaporate during the initial stages of the drying process while, under other conditions or with other compositions, essentially all VOC evaporate during the final stages of the drying process. Since it is desired to reduce the discharged amount of VOC, the VOC present in the process gases are destroyed, normally using either catalytic or thermal oxidation. In order to secure an efficient destruction of the VOC present in the process gases, the exhaust gases are heated to a temperature of approximately 350xc2x0 C. for catalytic oxidation, and 850xc2x0 C. for thermal oxidation. A required gas temperature for achieving sufficient oxidation is determined by the composition of the waste gases, which varies according to the composition, type and quality of the wood raw material. According to prior art, the waste gases are heated to the elevated temperature by means of a complicated and costly system, comprising heat exchangers of regenerating or recuperative type, where the heat of the waste gases, from which the VOC have been eliminated, is used for heating the gases still containing VOC, before they are delivered to the reactor. It is often, however, necessary to supply additional heat to the waste gases upstreams or in the reactor, in order to achieve the desired reduction of the VOC contents. According to prior art, the waste gases are vented from the drying device in only one place. This means that the whole amount of process gas leaving the drying device has to be preheated prior to oxidation in the reactor. This, together with the fact that the process gases often are heated to a higher temperature than necessary in order to secure an efficient reduction of the VOC discharge from the drying device, leads to increasing demands on, and size of, the heat exchange system arranged at the reactor, as well as an increased required energy for the destruction of VOC, and for the drying process as a whole.
The primary aim of the invention is to provide a method for drying wood raw material, comprising a destruction of VOC evaporated during the drying with a more efficient use of energy, and lower investment needs. The inventive method is cost efficient, with low maintenance requirements and a low energy needs, which secures the oxidation of essentially all of the VOC which evaporate into the drying gases prior to venting the waste gases into the atmosphere.
Another aim of the invention is to provide a method for drying wood raw material which, irrespective of variations regarding the type, composition and quality of the wood raw material, secures the oxidisation of all of the VOC evaporated into the drying gases before venting the drying gases into the atmosphere.
Further advantages of the present invention are apparent from the following description of the invention, and from the described preferred embodiments.
In order to achieve said results, the present invention suggests a method for drying wood raw material, where the wood raw material is dried in at least two sub-steps, the drying being controlled so that essentially all VOC are removed during one sub-step, and so that said VOC from the wood raw material is concentrated in the process gas which is removed during the sub-step, the gas being separated from the remaining process gases. The gas containing VOC is heated before it is transferred to a reactor where it is oxidised catalytically, at a gas temperature which exceeds 180xc2x0 C. A suitable gas temperature for the oxidisation of the VOC is from 200xc2x0 C. to 500xc2x0 C. Preferably the VOC is oxidised within a temperature interval from 200xc2x0 C. to 350xc2x0 C. The increase in gas temperature to a level which is necessary for catalytic oxidisation in the reactor is achieved by mixing the drying gases, with hot gases upstreams from the reactor, preferably combustion flue gases. By dividing the drying process into two or more sub-steps, while at the same time controlling the drying process so that the VOC is removed from the wood during one of the sub-steps, which leads to the concentration of essentially all of the VOC in the gases which are removed from the sub-step, the amount of gas to be treated is reduced significantly. At the same time, the VOC concentration in the amount of process gas to be treated is increased. Preferably the process gases, which no longer contain VOC, are returned to the drying process as drying gas. Process gas which has been treated in the catalytic oxidation reactor is supplied to the drying process as drying gas, downstreams from the reactor, in a different sub-step in the drying process.
According to a preferred embodiment of the invention, the wood raw material is dried in an essentially continuous process, comprising at least two sub-steps. Sub-flows of process gas are removed from the different sub-steps and the drying process is controlled so that essentially all VOC evaporate from the wood raw material during one sub-step, thereby concentrating essentially all the VOC to the sub-flow from said sub-step. The gas temperature in said sub-flow is increased before the VOC are removed by catalytic oxidisation at an elevated temperature in a reactor.
As was described above, the process gas containing VOC is heated before oxidisation, preferably by mixing it with hot gases upstreams from the reactor. According to a preferred embodiment of the invention, hot drying gases are used for increasing the temperature to a required level before oxidisation of VOC in the reactor. After oxidisation in the reactor, the process gas, now relieved from its VOC contents, is transferred to a further process step. Normally, the hot drying gases are generated by combustion of a suitable fuel, such as oil, gas or wood or other suitable bio-fuel. It may be suitable to use waste material from the on-site production facility in this fuel. As an alternative or complement to the combustion gases, it is of course possible to use other heat sources, such as gas heaters based on electrical resistance elements, heat exchangers for extracting excess heat from the drying process or, even better, from another process with waste gases with a high heat content.
It has, during application of the inventive process, turned out that in order to achieve an efficient decrease of the VOC contents in the process gas, the gas temperature should exceed 180xc2x0 C. It is advantageous to heat the process gas, before the oxidation, to a gas temperature in the interval between 200xc2x0 and 500xc2x0 C. Preferably a gas temperature in the interval between 200xc2x0 C. and 350xc2x0 C. is chosen, in order to secure the destruction of essentially all the VOC, without unnecessarily influencing the energy efficiency in a negative way.